At present, 3GPP (Third Generation Partnership Project) is working on standardization of the 4th generation mobile communication system. “LTE (Long Term Evolution)”, which is one of the data communication specifications defined by the 3GPP, is a long term advancement system aiming for 4th generation (4G) IMT-Advanced, and is also referred to as “3.9G (super 3G)”.
In the LTE, 2 types of duplex methods, FDD (Frequency Division Duplex) and TDD (Time Division Duplex) are available for selection. In the FDD, a band dedicated for uplink and a band dedicated for downlink are used. For each of the uplink and the downlink, a radio frame format including 10 contiguous subframes is used. The uplink herein refers to communication from a terminal station (UE terminal: User Equipment) to a base station of LTE (eNodeB: evolved Node B), and the downlink refers to communication from an eNodeB to a UE terminal. In the TDD, a radio frame format including 10 contiguous subframes is used, too. However, in the TDD, the same band is used for communication in the uplink and the downlink. Each subframe in a radio frame includes a control signal PDCCH (Phy Downlink Control Channel) from an eNodeB, and a PDSCH (Phy Downlink Shared Channel) which is used as user data.
In the LTE, 1-cell reuse is applied, that is, one frequency is used by all the cells in common. This is because using different frequencies between adjacent base stations as in a conventional cellular causes a shortage of frequency resources. In this case, a problem arises in that radio waves transmitted and received via UE terminals around a cell cause interference. Thus, the LTE, which is the 3GPP Rel 8, uses a technology called inter cell interference coordination (ICIC) in the Rel 8.
The ICIC can be achieved, for example, by fractional frequency reuse which is a combination of 1 cell frequency reuse and multiple cell frequency reuse. FIG. 14 illustrates the manner in which three cells 1 to 3, which perform fractional frequency reuse, are adjacent to each other. In FIG. 14, the range of each cell is indicated by a hexagon. In the fractional frequency reuse, each cell is divided into a central area (unshaded area inside the cell) which is inside the cell and near an eNodeB, and a peripheral area (shaded area at the edge of the cell) which is at the edge of the cell and located away from the eNodeB. Although a “center frequency” assigned to the communication between the eNodeB and UE terminals in the central area causes a conflict (that is, 1-cell frequency reuse) between the cell and an adjacent cell, the eNodeB avoids interference between the cells by reducing transmission power so that signals can be transmitted only within the central area. On the other hand, the eNodeB needs to use high power for transmission in order to send signals to the peripheral area, and avoids interference between the cells by using mutually different “peripheral frequencies” (that is, multiple cell frequency reuse) for the peripheral areas of the cell and the adjacent cell. In the illustrated example, a 20 MHz band is divided into, for example, 3 bands, and peripheral frequencies are reused so as not to overlap with each other between adjacent cells. In FIG. 14, differences between frequency bands are indicated by types of shading (diagonal line, vertical shading line, horizontal shading line).
In addition to the above-mentioned frequency reuse technology, in the ICIC of the 3GPP Rel 8, a signal for reducing interference is exchanged between base stations, that is, eNodeBs via the X2 interface. The X2 interface is an interface that connects between eNodeBs, and is typified by a transmission medium such as an optical fiber. Specifically, High Interference Indicator (HII) and Overload Indicator (OI) are each defined as a message to be exchanged via the X2 interface.
The HII is the information for informing an adjacent eNodeB of the location of a resource block which is assigned to a UE terminal at the cell edge. It is probable that the adjacent eNodeB is subject to interference from a resource block specified by the HII. Thus, taking this into consideration, the adjacent cell performs scheduling of the resource block. On the other hand, the OI is the information for informing of the level of interference of an uplink resource block, and has 3 levels Low/Medium/High. When the adjacent eNodeB is informed by the OI via the X2 interface that the level of interference to a certain resource block is High, the adjacent eNodeB adjusts scheduling of the resource block, and/or uplink power control.
In this manner, the ICIC in the 3GPP Rel8 adopts the adjustment method via the X2 interface for the purpose of eliminating interference between macro cells. The method, however, allows only the PDSCH in subframes to be adjusted, and the PDCCH portion cannot be adjusted. This is because the PDCCH has a format which allows the same band to be used between adjacent cells and is resistant to interference.
The ICIC in 3GPP Rel 10 will be described in the following. The ICIC in the Rel 10 aims to reduce interference between a macro cell and a pico cell.
In the 3GPP, a network referred to as HetNet has been studied in which various sized cells such as Macro/Micro/Pico/Femto are hierarchically structured to increase the overall system capacity. For example, a Pico eNodeB, which is the base station of a pico cell, has a characteristic that the transmission output thereof is lower than the transmission output of a Macro eNodeB which is the base station of a macro cell, of the order of tens of dB. It can be assumed that the X2 interface is provided between the Macro eNodeB and the Pico eNodeB (in other words, the interference of the PDSCH portion in subframes has been addressed by the ICIC in the Rel 8). However, in some cases, it is necessary to assume that the X2 interface between the Picoe NodeB and the Macro eNodeB has inferior characteristics of speed, capacity, and delay compared with the X2 interface between Macro eNodeBs.
Because the transmission power from the Pico eNodeB is low, an increasing number of areas will receive signals with higher power from the Macro eNodeB. Even in an area where a loss in transmission from a pico cell is lower than a loss in transmission from the Macro eNodeB (or an area which is closer in distance to the Pico eNodeB than the Macro eNodeB), higher received power from the Macro eNodeB often causes a UE terminal to attempt RRC (Radio Resource)_Connected to the Macro eNodeB in a far distance rather than the Pico eNodeB in the vicinity. However, uplink connection is advantageously made to a base station having a lower transmission loss in consideration of consumption of the battery at the UE terminal, and it is important to obtain a gain through cell division by assigning UE terminals to the pico cell in a heterogeneous environment such as HetNet including combinations of different types of cells. For these reasons, it is necessary to address the problem that each UE terminal tends to be connected to the Macro eNodeB only.
Thus, the Rel 10 defines a technology referred to as Range Expansion. The Range Expansion will be described with reference to FIG. 15. A UE terminal, when performing cell selection, that is, determining a base station to be associated with, selects to be associated with an eNodeB having higher power, based on the received power (RSRP: Reference Signal Received Power) obtained from a reference signal (Cell-specific reference signal) from each eNodeB. When the RSRP is evaluated for each eNodeB, for example, an offset of 10 dB is added to the RSRP of the Pico eNodeB for the evaluation so that the area including UE terminals to be associated with the Pico eNodeB is expanded. This is the Range Expansion and the area of expanded portion is called a Range Expansion Area. The Range Expansion Area is an area in which a UE terminal, which is originally to be associated with the Macro eNodeB due to low RSRP from the Pico eNodeB, can be associated with the Pico eNodeB thanks to an offset of RSRP, that is, the technology of the Range Expansion.
Some UE terminals in the Range Expansion Area may have higher received power from the Macro eNodeB than that from the Pico eNodeB which is associated with. In other words, the Range Expansion Area has a drawback in that the reception from the Pico eNodeB by the UE terminals is susceptible to interference from the Macro eNodeB. In the Range Expansion Area, a problem occurs of downlink interference between the Pico eNodeB and the Macro eNodeB.
For example, a communication system which controls interference to a home base station device has been proposed, the communication system including a mobile station, a base station device that manages macro cells, and the home base station device that manages a femto cell, a pico cell, a nano cell, and a home cell (for example, refer to Patent Literature 1). However, the communication system does not control downlink interference to the UE terminals in the Range Expansion Area.